1. Field of the Invention
The present invention relates to a liquid droplet ejection head, such as an ink jet head mounted on an ink jet printer, including a passage unit, and a method for manufacturing the liquid droplet ejection head.
2. Description of the Related Art
An ink jet head that is one example of a liquid droplet ejection head includes a passage unit and an actuator. Liquid passages are formed in the passage unit, and are constituted by: a plurality of nozzles configured to eject liquid droplets; a common ink chamber connected to a liquid supply source; a plurality of pressure chambers communicated with the common ink chamber; and an outflow passage which causes the pressure chamber and the nozzle to be communicated with each other. The passage unit is constituted by stacking a plurality of plates having holes and/or grooves which form the liquid passages, and is structured such that a manifold plate forming the common ink chamber and other plates are disposed as intermediate layers between a pressure chamber plate forming the pressure chamber and a nozzle plate forming the nozzle (see Japanese Laid-Open Patent Application Publication 2004-25636 for example). The plates other than the nozzle plate are generally metal plates.
In accordance with this ink jet head, the actuator selectively drives to apply an ejection pressure to an ink stored in the corresponding pressure chamber. Thus, ink droplets are ejected outside from a nozzle hole communicated with this pressure chamber, and the ink is supplied from the common ink chamber to the pressure chamber by a negative pressure.
The ejection pressure generated in the pressure chamber selected at this time contains not only forward components transmitted toward the nozzle hole but also backward components transmitted toward the common ink chamber. Therefore, the pressure may reach the other pressure chambers by the backward components through the common ink chamber, i.e., so-called cross talk may occur. On this account, the passage unit includes as the intermediate layer a damper plate stacked on the manifold plate. Formed at a portion of the damper plate which portion corresponds to the common ink chamber are: a damping space formed by forming a recess on a surface of the damper plate which surface is opposite a surface facing the common ink chamber; and a thin damper wall. By stacking the other plate on the damper plate such that the damping space is closed, a damper chamber is defined by the damper wall separately from the common ink chamber. In a case where the backward components of the ejection pressure are transmitted backward from the pressure chamber to the common ink chamber, pressure fluctuations caused by the backward components are absorbed by elastic deformation of the damper wall, and this damps the pressure fluctuations in the common ink chamber.
To improve such damper effect, it is contemplated that, for example, the area of the damper wall is increased. However, it has been contemplated in recent years that the flexibility of the damper wall is further improved since the ink jet head has been reduced in size.
To obtain the high damper effect in the case of a metal damper plate used in Japanese Laid-Open Patent Application Publication 2004-25636, it is contemplated that the thickness of the damper wall is reduced. However, an ingot of a metal material that is a raw material of the damper plate contains a small amount of impurities including an additive injected thereto at the time of manufacturing. Therefore, in a case where the thickness of the damper wall is reduced to be the same level as the particle diameter of the impurity, the impurity in the damper wall may fall off, and this may cause a partial damage or crack on the damper wall. If such damage or crack forms a through hole on the damper wall, and the common ink chamber and the damper chamber are communicated with each other by this through hole, liquid may intrude from the common ink chamber to the damper chamber, or air may intrude from the damper chamber to the common ink chamber.
Moreover, Japanese Laid-Open Patent Application Publication 2006-347036 proposes the passage unit in which the damper plate is constituted by a metal base portion and a resin sheet, and the resin sheet is interposed between the base portion and the manifold plate. Herein, a portion corresponding to the damper wall is the resin sheet.
However, in a case where the portion corresponding to the damper wall is made of a resin material as in Japanese Laid-Open Patent Application Publication 2006-347036, the resin material may allow a gas to be transmitted therethrough by its nature. To maintain meniscus of the ink in the nozzle, an ink passage in the ink jet head is normally maintained in a negative pressure state. Moreover, since the damper plate is constituted by the metal base portion and the resin sheet, and the resin sheet is stacked on the entire surface of the base portion, a hole that forms the outflow passage needs to be formed on the resin sheet so as to penetrate the resin sheet. Therefore, the outflow passage increases in length by the thickness of the resin sheet, and this increases a passage resistance of the outflow passage. On this account, to secure an adequate ejection pressure for ejecting the ink droplets from the nozzle, the drive voltage of the actuator needs to be increased. Moreover, to achieve a desired liquid passage resistance of the outflow passage, it is necessary to accurately process a cross-sectional shape of the outflow passage, so that the step of processing the outflow passage with respect to the resin sheet may require time, labor, and cost.